How do I properly install an air filter in a furnace?

To properly install an air filter in a furnace, first turn off the furnace. Locate the filter slot, usually near the return air duct. Remove the old filter and note the direction of airflow indicated on it. Insert the new filter with the airflow arrow pointing towards the furnace. Make sure the filter fits snugly and securely in place. Turn the furnace back on and check for proper airflow.